Soil Temperature and Seed Germination

When I worked as a garden teacher in Baltimore City, I would hear the following question every spring: "How do seeds know it's spring? How do they know it is time to start growing?"

What a wonderful question, one that really gets to the heart of our relationship with nature's grandest displays—the cycling of the seasons.  While light is an important factor for some seeds, soil moisture must be adequate, and the factor that most often serves as nature's "alarm clock" in the spring is soil temperature. For every type of plant, there is an optimum temperature for germination. Some are adapted to cool weather climates (spinach, kale), and some come from warmer climes such as watermelons and tomatoes. A cool-season crop like lettuce can germinate with soil temperatures just above freezing, while a tomato seed won't even think about starting to germinate if the soil temperature is below 50°F.

For many garden plants in fact, a soil temperature below 50°F can be a problem. With temperatures in the 40s, seeds will readily absorb water, but not start to grow. This creates an opportunity for disease and rot, which can result in damping-off or poor growth. Many native plants require a specific duration of cold temperatures before they will germinate- that way they don't start growing on a balmy November day only to be frozen in December. This is known as "cold stratification," a process we sometimes emulate by placing seeds in a plastic bag of moist perlite in the fridge several weeks before planting.

The astounding flexibility and adaptability of life doesn't end here, however! Some seeds have developed a more nuanced approach to germination than one dictated only by temperature. The seed coat of some seeds needs to be broken before they will germinate. Certain seeds require help from others—they may need to be scraped up in the guts of a bird before being deposited (along with some of the bird's homemade fertilizer) in the soil. This process is called "scarification." When seeds need to be scarified, gardeners will use sandpaper, the edge of a coin, or soak the seeds in warm water to achieve the desired effect.

How can we put knowledge of soil temperatures to work in our own gardens? When starting seeds indoors, don't assume that they all need the same conditions. All seeds have an ideal soil temperature, which you can achieve with a heated propagation mat. Covering the flat with a clear dome afterwards helps to hold in heat and moisture. A soil thermometer can be a good investment if you are having trouble getting seeds to grow, they are only a few dollars and are well worth the expense.

Outside in the garden, there are things we can do to start our growing season off right. Soils respond to the rapidly shifting conditions of spring differently than air does. As the dark color of bare soil absorbs more heat, some of that heat rises to warm the air. The rest of the sun's energy makes its way down through the soil. A soil's highest temperature at a depth of 2 inches will be about 33% less than the surface temperature, while at 4 inches the temperature is 66% less than the surface temperature. At night, however, the deeper soils hold on to heat longer than the air. This means that soil temperatures are more stable than air temperatures, which is great for seedlings. It also means that the soil takes a longer time to heat up (or cool down) than the surrounding air. The presence of water can emphasize this effect further—well-drained soils will heat faster and cool faster, heavy soils that retain water will take longer to warm in the spring but will hold that heat longer in the autumn. Many people like to get an early start in their garden by warming the soil. If you have a mulch on your soil, you can rake back the mulch temporarily in spring. The dark soil will absorb the light and convert it into heat, warming the soil. Another alternative is to cover the bed with a plastic sheet or glass pane, which can really heat things up on a sunny day.

Let's return to my opening question. How do seeds know when to start growing? While there are many factors at work, some seeds are nudged out of dormancy by the consistently warmer soils of spring. After reading this article, I hope you have learned a few ways to ensure your seeds don't sleep through nature's alarm clock. With a little extra work, you may even be able to get them out of bed a little early for once!

For more information and a chart providing minimum and ideal temperatures for common crops, visit Seed and Seedling Biology.